The dry, wheezy cough of a recurrent airway obstruction (RAO) sufferer is all too familiar in winter, when cold, wet conditions mean that all but the hardiest of horses are spending most of their time indoors.
It’s this indoor lifestyle — rather than the season alone — that can cause the common respiratory condition to flare up at this time of year.
A horse with RAO will have inflammation and increased accumulation of mucus in the airways, triggered by exposure to the dust, ammonia, moulds, dust mites and fungal spores commonly found in stables.
Stabling causes airway inflammation even in healthy horses, but the response is exaggerated in those with RAO. The mucus becomes stickier and less easy to clear, similar to human asthmatics. Airways also become constricted, reducing airflow and increasing the breathing effort.
The perils of pollution
Research from 2007, which involved a questionnaire survey of UK horse owners, identified exposure to this “urbanised environment” as a significant risk factor for RAO. This is most likely due to a combination of increased stabling and air pollution — pollution is certainly implicated in asthma in humans.
Other risk factors included increasing age (with an average age at diagnosis of 11 years), exposure to hay and history of a respiratory infection in early life.
The resulting clinical signs include exercise intolerance, cough, nostril flare, wheezing, respiratory distress and increased abdominal effort, characterised by marked muscle appearance known as a “heave line”.
Bouts of coughing are common and affected horses may have episodes of acute crisis followed by periods of disease remission.
Diagnosis is based on an assessment of the horse’s health history and a clinical examination, often backed up with endoscopy and collection of fluid from the trachea and lungs. Routine blood tests are usually normal and therefore rarely helpful.
Easing the airways
Once diagnosed with RAO, a horse is susceptible for life. Control of his environment is vital and may be all that is required for a significant improvement, although note that it can take up to four weeks for remission of clinical signs.
A severely affected horse may need to be kept entirely at pasture with provision of a shelter. Some very sensitive horses only need brief exposure to hay to cause clinical signs — even bringing such a horse inside for grooming may trigger an allergic response.
Because the inflammation induced after even a short exposure to dust can last for weeks, many affected horses need additional medical treatment to control signs and provide relief.
The aims of therapy for RAO are to reduce both the spasm in the airway and the inflammatory response. Bronchodilators alleviate the spasm to provide immediate relief and can be given intravenously, orally or via inhalation.
Corticosteroids reduce the inflammation and are the mainstay of long-term therapy. They too can be administered by intravenous injection, oral medication or inhalation devices. Inhaled corticosteroids have the advantage of being administered directly into the respiratory tract, requiring a lower dose and reducing the risk of side effects.
Aerosol sprays, such as those used by human asthmatics, are commonly prescribed (pictured top). They require careful administration for full effect, however, and are not well tolerated by all horses.
An equine nebuliser is often more effective. This inhalation mask, similar to those used in human medicine, delivers a mist of medication, without the hissing noise produced by inhalers to which some horses are particularly adverse. As such, it is a well-tolerated treatment option.
Steam or soak?
If there’s no alternative to winter stabling, how can you control the condition? The main aim should be to keep his immediate environment dust-free and well ventilated. Stable him away from forage stores and always remove him from the area when you muck him out.
Hay and bedding are the main sources of the trigger substances, such as dust. The horse’s movements around the stable will increase the amount of airborne particles — particularly in the “breathing zone” immediately around his nostrils.
Even good-quality hay contains large amounts of dust and moulds. When hay is obviously dusty and has visible mould, the amount inhaled by the horse will be enormous. Haylage generally has much less dust and may be better tolerated.
Soaking hay can reduce the amount of dust, but this may not be sufficient to reduce RAO signs and will not remove the mould and spores.
Studies have shown that steaming significantly reduces the amount of bacteria, fungi and yeasts in hay, whereas prolonged soaking can actually increase bacterial colonies. Steamed hay is also far less likely to induce clinical signs in RAO-susceptible horses than dry hay. Even so, dampened pelleted forage may need to be fed to some very sensitive horses.
With regards to bedding, some shavings can contain even higher levels of dust than good-quality straw. A better alternative is dust-extracted shavings, paper or cardboard, or rubber matting. Deep litter is not suitable as it allows a large build-up of moulds and fungi.
There is no cure for RAO, but early diagnosis and appropriate management can allow horses to remain competitive for many years and may even result in lifelong remission.
Is ROA inherited?
Recent research from the past three years in Switzerland has shown a strong genetic predisposition for the development of RAO. Offspring from affected parents therefore have a significant risk of developing the disease as adults.
Unfortunately, the genetics are quite complicated and involve several genes, so a single genetic test is not available. The affected genes are not identical in each RAO-susceptible horse; individuals may be allergic to different environmental factors, but the overall clinical signs are the same.
Additionally, there is a strong association between the risk of developing RAO and increased risk of the same horse developing allergic skin disease such as insect bite hypersensitivity (sweet itch). It is also suggested that exposure of horses to hay at a young age may increase their likelihood of developing RAO in the future.
This new and really exciting area of research may help future breeding programmes reduce the number of horses affected by this potentially debilitating disease.
What’s in a name?
RAO is the new name for COPD (chronic obstructive pulmonary disease). The name change occurred because COPD in humans is an irreversible disease, commonly related to smoking — clearly not an issue for horses.
RAO more closely resembles human asthma and is an exaggerated inflammatory response to hay and stable dust, predominantly affecting horses over seven. It can cause significant distress to the affected horse, as well as considerable expense in terms of treatment and long-term management. It is an important cause of poor athletic performance.
Signs of RAO may not only occur when stabled. There is a summer form of the disease associated with inhaled pollens and dusts at pasture. This can be more problematic to control than simple RAO and often needs long-term treatment and removal from pasture. The two forms of the condition can occur together, making management very difficult as both stabling and turnout cause clinical signs.
Managing winter coughs: a case studies
For Ben, a 13-year-old riding school cob with limited available turnout, minimal exposure to dusts is key.
Ben was diagnosed with RAO after developing a summer cough that was exacerbated by exposure to smoke from a neighbouring farmer’s field. Despite initial treatment, he continued to show periods of increased respiratory rate and effort when stabled. Endoscopy showed increased mucus in his trachea; a lung wash sample revealed significant inflammation and confirmed RAO.
Ben was treated with bronchodilators to allow easier breathing, along with corticosteroids. He is now back in work and his environment is carefully managed to reduce his exposure to dusts, moulds and pollens.
With two competition horses prone to winter dust allergy problems, Alison Payne is keen to keep her yard as dust-free as possible.
“They’re fine while they’re out in summer, but in winter the coughing and runny noses can start,” said Alison, who uses rubber stable mats and low-dust bedding for Langarth Delia and Rarumba. She also turns them out as much as possible in winter and steams their hay in a Haygain steamer.
Research at the Royal Agricultural University in 2010 showed that steaming hay at the high temperatures achieved in Haygain steamers kills all bacteria and mould and reduces the respirable particle content by more than 95%.
This Veterinary Clinic feature was first published in Horse & Hound magazine (12 December, 2013 edition)
